Fix export when MPI is not activated.

experimental/cmake/export.cmake

@@ -2,7 +2,11 @@
 # Export Library
 # --------------
 
-set(PACKAGES ${CMAKE_PROJECT_NAME}-headers ${CMAKE_PROJECT_NAME}-core ${CMAKE_PROJECT_NAME}-mpi)
+set(PACKAGES ${CMAKE_PROJECT_NAME}-headers ${CMAKE_PROJECT_NAME}-core)
+
+if(${CMAKE_PROJECT_NAME}_USE_MPI)
+    list(APPEND PACKAGES ${CMAKE_PROJECT_NAME}-mpi)
+endif()
 
 install(TARGETS ${PACKAGES}
         EXPORT ${CMAKE_PROJECT_NAME}-targets